An illegal fish trap cage was found in the Marina Reservoir on Tuesday by an otter watcher who spotted a group of otters exploring the device.
She alerted the national water agency PUB, which removed the trap, and issued a call for witnesses.
The otter watcher spotted the otters climbing on a structure near the Indoor Stadium, opposite Tanjong Rhu, and realised it was a trap for animals. She informed PUB, which removed the trap from the reservoir with the help of community group OtterWatch.
PUB, in a Facebook post on Tuesday, said no animals were found in the trap. It is illegal to use such traps in reservoirs or waterways, even in areas where fishing is allowed.
Those caught trapping animals or doing something that injures animals in reservoirs can be fined up to $3,000.
Mr Adriane Lee, a volunteer zone captain with International Coastal Cleanup Singapore, yesterday told The Straits Times that the contraption was a funnel trap used to trap fish.
He said: "The idea is for fish to enter in one direction and they can't get out as they don't know how to find their way out," said the 43-year-old manager.
International Coastal Cleanup Singapore helps to clean up Singapore's coastlines and waterways.
Mr Lee, who has been a zone captain for six years and oversees the north-western parts of Singapore, said trapped fish can attract larger predators such as otters and birds, which can then get trapped themselves.
He said: "For example, an otter can get its head stuck inside the hole. If the trap is lost or abandoned - for example, if the owner forgets where he laid the trap or if water currents shift it - it is even worse, as it will be in the water, trapping (animals) indefinitely."
Mr Kalai Vanan, deputy chief executive at Animal Concerns Research and Education Society (Acres), said the group often responds to cases where animals become entangled or get caught in cages, traps or fishing lines.
"We urge the public to be vigilant when walking along park connectors and waterways, and to inform the authorities if they see such traps, fishing lines or people fishing in prohibited areas."
The public can call the PUB hotline on 1800-CALL-PUB (1800-2255-782) if they have information on a case, or if they spot suspicious activities at reservoirs.